A network-based ID system monitors the traffic on its network segment as a data source. This is generally accomplished by placing the network interface card in promiscuous mode to capture all network traffic that crosses its network segment. Network traffic on other segments, and traffic on other means of communication (like phone lines) can't be monitored. Both network-based and host-based ID sensors have pros and cons. In the end, you'll probably want a combination of both.
Network-based ID involves looking at the packets on the network as they pass by some sensor. The sensor can only see the packets that happen to be carried on the network segment itís attached to. Packets are considered to be of interest if they match a signature. Three primary types of signatures are string signatures, port signatures, and header condition signatures.
String signatures look for a text string that indicates a possible attack. An example string signature for UNIX might be "cat "+ +" > /.rhosts" , which if successful, might cause a UNIX system to become extremely vulnerable to network attack. To refine the string signature to reduce the number of false positives, it may be necessary to use a compound string signature. A compound string signature for a common Web server attack might be "cgi-bin" AND "aglimpse" AND "IFS".
Port signatures simply watch for connection attempts to well-known, frequently attacked ports. Examples of these ports include telnet (TCP port 23), FTP (TCP port 21/20), SUNRPC (TCP/UDP port 111), and IMAP (TCP port 143). If any of these ports arenít used by the site, then incoming packets to these ports are suspicious.
Header signatures watch for dangerous or illogical combinations in packet headers. The most famous example is Winnuke, where a packet is destined for a NetBIOS port and the Urgent pointer, or Out Of Band pointer is set. This resulted in the "blue screen of death" for Windows systems. Another well-known header signature is a TCP packet with both the SYN and FIN flags set, signifying that the requestor wishes to start and stop a connection at the same time.
Well-known, network-based intrusion detection systems include AXENT (acquired by Symantec), Cisco (www.cisco.com), CyberSafe (www.cybersafe.com), ISS (www.iss.net), and Shadow (
A good ID capability will use both host- and network-based systems. Figuring out where to use each type and how to integrate the data is a real and growing concern.